Civil Servant (IAS/PCS)

Indian polity is a crucial segment of the UPSC Prelims syllabus.
If you take political science as your optional subject, you will study for this optional along with the prelims section thus saving time for other topics. This way you can integrate the UPSC prelims and mains preparation.
Even if you don't have political science as your background, you can easily comprehend and study this subject as it is not too technical. The concepts are general and anyone who reads newspapers on a regular basis would be able to understand and prepare for it.
In the new revised syllabus, the factual parts have been omitted thus an understanding of the concepts is only required to write better answers in this paper.
The notion that the syllabus for political science is vast is only partially true. In fact, a lot of the topics are repeated like Marxism, Marx, Marxist notions of state, Gramsci, etc.
Many topics are interrelated like thoughts of Mill, Bentham and Liberal notions of state, justice and democracy and so on.
Topics such as liberalism, feminism, ideas of Marxism, issues like democracy, human rights, environment, etc. are covered in both Paper I and Paper II.
Knowing a lot of content in this subject can also help in your current affairs preparation.
In the interview round also, a good knowledge of topics in polity such as governance, international relations, etc. can be very handy.